1. Field of the Invention
The present invention relates to a Coded Orthogonal Frequency Division Multiplex (COFDM) demodulator.
2. Discussion of the Related Art
FIG. 1 is intended to illustrate the principle of a COFDM modulation. Data packets to be transmitted are put in the form of N complex coefficients associated with N respective frequencies (or carriers). Number N of the frequencies is equal, for example, to 1705 for the so-called “2K” mode and to 6817 for the so-called “8K” mode, in digital television radio transmission. Each complex coefficient corresponds to a vector which is illustrated in FIG. 1 as starting from a frequency axis at a point indicating the frequency associated with the coefficient.
The set of these N coefficients is processed by inverse fast Fourier transform (IFFT), which generates a “symbol” formed of a sum of modulated carriers, each carrier having an amplitude and a phase determined by the associated complex coefficient. The symbol thus generated is transmitted.
Conventionally, in radio transmission, the width of the transmission channel is 6, 7, or 8 MHz and each carrier is separated from the next one by a frequency difference Δf=1/Tu. Tu is the transmit time of a symbol and is called the useful duration. The useful duration is on the order of 224 μs in 2K mode and 896 μs in 8K mode, for a 8-MHz passband.
Upon reception, a receiver submits the symbol to the inverse processing, that is, mainly, a fast Fourier transform (FFT) to restore the initial complex coefficients.
As shown in FIG. 1, certain regularly distributed vectors P1, P2, P3 . . . have a known constant value. These vectors, or the corresponding carriers, are said to be pilot. They are used to reflect the distortions undergone by the transmitted channel and by the information that they provide on the channel response, they enable correcting the unknown vectors located between the pilots.
FIG. 2 illustrates a transmission of several successive symbols Sn−1, Sn . . . . As shown, each of these symbols is preceded by a guard interval Tgn−1, Tgn, which is a copy of a portion of the end of the corresponding symbol. The guard intervals are often defined by a fraction of useful time period Tu. Conventional values of the guard interval are Tu/32, Tu/16, Tu/8, or Tu/4.
The guard intervals are used to avoid inter-symbol modulation distortions caused by an echo of the transmission. FIG. 2 also shows an echo SEn−1, TgEn−1 . . . of the transmitted signal. This echo is delayed with respect to the main signal by a time period shorter than that of a guard interval Tg.
Each symbol S is normally analyzed by the FFT circuit of the receiver in a window W, or FFT analysis window, of same length as the symbol. If there was no guard interval, an analysis window W coinciding with a symbol of the main signal would be astride two symbols of the echo signal. This would cause an error which would be difficult to correct in the calculation of the FFT Fourier transform.
Guard interval Tg, provided that it is greater than the delay or the advance of the echo, provides an adjustment margin for analysis window W so that it only coincides with portions belonging to the same symbol, in the main signal as well as in the echo. The fact for an analysis window to be astride a symbol and its guard interval introduces a phase shift which is corrected by means of the above-mentioned pilots.
In FIG. 2, symbol Sn−1 must be analyzed in a window Wn−1 of duration Tu that can be placed indifferently in a window delimited by times ta and tb, time ta corresponding to the beginning of the guard interval of echo TgEn−1, and time tb corresponding to the end of symbol Sn−1. Similarly, symbol Sn must be analyzed in a window Wn of duration Tu that can be positioned indifferently in the window delimited by times tc and td, time tc corresponding to the beginning of the guard interval of echo TgEn, and time td corresponding to the end of symbol Sn.
FIG. 3 schematically shows the place of pilots in the symbols. The symbols are gathered in frames of 68 symbols, conventionally in digital television radio transmission (standard ETSI EN 300 744, V1.4.1).
In FIG. 3, each line represents a symbol and each box represents the position of a carrier. The carriers are defined as going from a position 0 to a position Kmax, Kmax being equal to 1704 in 2K mode and 6816 in 8K mode. Indeed, only a portion of the possible frequencies is used, especially due to risks of losses at the channel border. The pilots are of two types.
On the one hand, there are, in each symbol, continuous pilots Pc. The continuous pilots correspond to specific frequencies distributed in the channel. In the above-mentioned ETSI standard, there are 45 in 2K mode and 177 in 8K mode. The continuous pilots are present in all the symbols and always take up the same frequency position. In FIG. 3, only the continuous pilots corresponding to positions 0 and Kmax have been shown.
On the other hand, there are, in each symbol, so-called “scattered pilots” Pr, which are arranged every 12 carriers, and shifted by three positions between two successive symbols. Thus, every four symbols, the same arrangement of scattered pilots Pr can be found.
Initially, the FFT analysis windows are roughly positioned, for example, by a method of intercorrelation of the received signal. The continuous and scattered pilots, of constant amplitude on transmission, are then used to finely position analysis windows W.
For this purpose, at the receiver, the complex time received signal, after having been put in baseband, is provided to a fast Fourier transform unit providing the symbol in the frequency field. The pilots are sampled from this symbol. The sampled pilots enable estimation of the frequency response of the channel which, after having undergone an inverse Fourier transform, provides the estimation of the channel pulse response. The estimation of the channel pulse response is used to finely position analysis window W.
However, a problem exists due to the fact that the pilots provide but an incomplete description of the channel. Indeed, a taking into account of several successive symbols (at least 4) only enables having an image of the channel every three points. As a result, the estimation of the pulse response of the channel obtained from the pilots exhibits a periodization of period equal to useful duration Tu divided by three.
FIG. 4A shows an example of a curve 1 representative of the module estimation of the frequency response of a transmit channel. Black disks 2 correspond to the points of curve 1 available from the continuous and scattered pilots of four successive symbols. The estimation of the pulse response of the transmit channel is obtained by the inverse Fourier transform (IFFT) of the points of the estimation of the frequency response obtained for the continuous and scattered pilots.
FIG. 4B shows the module estimation of the pulse response of the channel obtained from the points of the module estimation of the frequency response shown by the black disks in FIG. 4A. The module estimation of the pulse response is schematically shown by a periodic series of pulses 4 having a period equal to useful duration Tu divided by 3. For each period, a pulse corresponds to the main path taken by the signal and the other pulses correspond to echoes. In practice, the estimation of the pulse response corresponds more to a periodic series of “blunted” peaks.
FIG. 5 schematically illustrated the steps of a conventional method for positioning an analysis window of the FFT circuit based on the module estimation of the pulse response. The method comprises the steps of:
searching, over a period of the module estimation of the pulse response, pulse 7 of maximum amplitude which corresponds to the main path;
displacing a window FE, having its width corresponding to guard interval Tg, with respect to main path 7 from an initial position 8 (shown in dotted lines) to a final position 9 (shown in full line) and determining, for each position of window FE, the “energy” of the module estimation of the pulse response in window FE, the “energy” for example corresponding to the sum of the amplitudes present in window FE; and
refining the positioning of analysis window W from the position of window FE corresponding to the maximum “energy”.
The position of window FE corresponding to a maximum energy is generally further used to determine the estimation of the frequency response for the carriers other than continuous or distributed carriers, represented by white disks 6 in FIG. 4A. For this purpose, an interpolation filter is generally applied to the estimation of the pulse response, the positioning of which is refined according to the maximum energy position of window FE.
Such methods to refine the positioning of the FFT analysis window and the positioning of the interpolation filter are implemented on the demodulator referred to as STV0360 sold by the applicant.
In the previously-described method for positioning the FFT analysis window, the width of window FE is equal to guard interval Tg. This means that all the echoes outside the guard interval are not taken into account to refine the positioning of the FFT analysis window. However, echoes are generally present outside of the guard interval. The fact for them not to be taken into account to refine the positioning of the FFT analysis window may alter the quality of the demodulation. However, the width of window FE cannot be increased beyond the guard interval since this risks making the FFT analysis window positioning method unstable.